Permanent magnet and method of making the same



Sept. 11, 1956 J. J. WENT ETAL PERMANENT MAGNET AND METHOD OF MAKING THE SAME Filed July 30, 1951 JAN JACOBUS WENT GERARD WILLEM VAN QOSTERH'0UT EVERT WI%LLEM ORTER BY AGENT United States Paterit PERMANENT MAGNET A'ND METHOD OF MAKING THE SAME Application July 30, 1951, Serial No. 239,264

Claims priorty, application N etherlauds September 19, 1950 19 Claims. (Cl. 252-625) 'Ihis invention relates to ferromagnetic materials, to permanent magnets made therefrom and to methods of making the same.

One of the important requirements in the manufacture of ferro-magnetic materials is that the mater-ial be substantially free of impurities since such impurties tend to lower the values of the magnetic characteristics of the material. Consequently, extreme care in the seleetion of the basic constituents and in the processing of the same into the ferromagnetic material is essential.

Another important requirement, particularly for permanent magnet materials, is a high coercive force. Although permanent magnets have been developed which exhibit fairly large coercive forces; e. g. several hundred or more oersted, such magnets have the disadvantage that the raw materials used, for instance cobalt, are relatively expen-sive.

The main object of this invention is to provide ferromagnetic materials which are manufactured trom inexpensive raw m-aterials, are inexpensive to manufacture and at the same time have high coercive forces.

Another object of this invention is to provide a method of manufacturng ferr-omagnetc materials which can be magnetized into permanet magnets.

A further object of this invention is to provide a method of manufacturing low-cost ferromagnetc materials using inexpensive, relatively abundant raw materials.

A stil] further object of this invention is to provide a ferro-magnetic material suitable to be magnetized into a permanent magnet which has a high intrinsic coercive orce combined with a fairly high value of magnetic remanence.

And yet another object of this invention is to provide -a ferromagnetic material in which the presence of impurities in substantial amounts does not adversely atect the magnetc properties of the material.

Addi-tional objects of the invention will appear as the specificaton progresses.

The ferromagnetic materials according to the invention compnise essentially non-cubic crystals of polyoxides of iron and at least one of the metals selected from the group consisting of barium, strontium and lead. In addition, the materials may contain calcium in an amount equal to 2111 atomic fraction of up to 0.4 of one of the metals barium, strontium and lead. These materials have an intrinsic coercive field strength I c exceeding about 700 oersteds, a remanence Br of at least about 1200 gauss and a (BH) max of the order of 1.1 X

The ter-romagnetic materials according to the invention are prepared by forming a mixture of ferric oxide (Fe2Os) and one or more of the oxides of strontium, lead, barium and calcium in proportions which produce non-cubic crystals i. e., 21 mol ratio of between about 2:1 and 10:1, and heating the mixture to a temperature in a range of about 900 to 1450 C., preferably between about 1100 and 1450 C., and for suficient time to 2,762777 Patented Sept. 11, 1956 form the crystals of the polyoxides of iron and other met-als. When using -two or more of the oxides of cal cium, barium, lead and strontiurn mixed with Fez0a.and heated, non-cubic mixed orystals of the polyoxides of iron and these metals are formed.

The heating may be efiected.in various atmospheres provided they do not reduce the oxides to the metals or to unwanted lower oxides. We prefer to use air as the atmosphere, although one has considerable latitude as regards the choice of the gas atmosphere, always provided that no undue reduction is etfected.

The crystals formed by heating the mixture are noncubic in structure and, as a rule, 'are hexagonal. A specific class among the same is charaoterized by the crystal st-ructure of the mineral magn6t-oplumbite (Pb0.6 Mn, Fe)z0s) and by a chemical composition given by the formula MFe12O19, M being one or more of the metals bariu-rn, strontum and lead, as the case may be partially replaced by calcium to the extent indicated above for any one of said metals. A fur-ther specific class of crystals is characterized by a unit cell having axes of 5.8 A. (a-axis) and 33 A. (c-axis) and by a chemical composition given approximately by the formula MFC1BOZ'T, M having the same meaning as indicated heretofore; as may be inferred trom the formula in question a small portion of the 1'ron contained in these crystals is present in the bivalent state.

The materials are somewhat porous, the porosity depending mainly upon the length of time they were heated and the temperature atWhich they were heated. Very dense materials usually have a high magnetic remanence While the more poreus materials have a higher nternal coercive field strength so that the heating temperature andtime of heating must be selected to give the desired properties. When using lead as one of the consttuents, it is preferable, because of the volatility of this metal, to heat the mixture at a temperature in the lower part of the range and to use a somewhat larger quantity of the lead oxide constituent than appears necessary for the formation of the crystals.

The materials according to the invention have-a high volume resistivity, e. g. hgher than 10 ohms/cm. and When used as core materils need not be laminated t0 reduce eddy-current losses. Permanent magnet bodies according to the invention are relatively -dense bodies consisting essenti-ally of these materials. Such bodies are formed by compacting a finely-ground mixture of the raw materials followed by sintering. By sintering a mixture which has been compacted, not only a high density is obtaned in the sintered product but the reaction will be more complete with an accordingly higher yield of the desired ferrornagnetic compound. This leads to a higher value of saturation magnetization and therefore again to a high value of remanence. However, sintering to a dense body results in crystal growth and accordingly in a decrease of the coercive force.

The invention will be described in more detail with reference to the following illustrative examples and the drawings in which Figures 1 to 3 are X-ray difiraction photographs (powder patterns) of materials according to the invention. The micro-structur of the materials has been determined frorn X-ray ditfraction patterns of single crystals. Once said structure is known, the accompanying powder pattern canbe indexed so as to find out the dimensions of the unit cell.

gms (2.5 mols) of Fez0s (percent by weight Fe=69.8) was g1ound for 13.5 =hours with alcohol in a hall mill.

After being dried and sieved, 300 gms. of the mixture were mixed with a suitable binder, such as nitrocellulose or polyvinylalcohol and formed into a kneadable mass trom which rods of about 3 mms. in diameter were extruded. The rods were dried for about 24 hours in air and subsequently passed through an electric oven whose heating zone had a length of 10 cms. and had a temperature of alaout 1310 C. at -a speed of 5 mms./=min., so that the sintering time was 20 minutes. The sintering process took place in air. After being passed through the oven, the rod was cooled and had a remanence Br and an intrinsic coercive field strength 1% of 1200 Gauss and 1940 oersted respectively, measured at room temperature. The rod consisted of two phases, namely, crystals mainly 35 u. in size of BaFerzm and Ba0.Fez0s.

Example 2 In a simlar manner as described in Example 1, a mixture consisting of 201.1 gms. (1 mol) of barium carbonate (percent by weight of Ba=68.3) and 480.6 gms. (3 mols) of FezOz (percent by weight of Fe=69.8) was ground, dried and sieved, subsequently mixed with a binder and formed into a kneadable mass which was extruded into rods of 3 mms. in diameter. The rods were sintered in the oven mentioned in Example 1 at a speed of mms. per minute and at a temperature of 1330" C. As in Example 1, the sintering process took place in air. The length of the heating zone being cms. the sintering period was 20 minutes. The rods thus 0btained had a remanence B;- of 1510 Gauss and an intrinsic coercive field strength 1% of 2080 oersted, measured at room temperature.

Example 3 A mixture consisting of 201.1 gms. (1 mol) of barinm carbonate (percent by weight of Ba=68.3) and 560.7 gms. (3.5 mols) of Fez0a (percent by weight of Fe=69.8) was ground, dried sieved and extruded into rods as indicated in Example 2. The rods were sintered in air at 1330 C. for 20 minutes. The rods obtained had a remanence B1 of 1600 Gauss and an intrinsic coercive field strength 1 0 of 1990 oersted, measured at room temperature. crystals mainly 38/.L in size of BaFem0rg and a second phase identified as Fez0s.

Example 4 A mixture consisting of 99.2 gms. (.5 mol) of barium carbonate (percent by weight of Ba=69.2) and 564.8 gms. (3.5 mols) of Fez0s (percent by weight of Fe=68.4) was ground for 15.5 hours with alcohol in a ball mill. After being dried and sieved, 300 gms. of the mixture were mixed with a binder such as nitrocellulose or polyvinylalcohol, formed into a kneadable mass and extruded into rods, similarly as in the preceding example. The rods were then sintered in air in an oven at a tem perature of 1330 C. for 10 minutes. The rods thus produced had a remanence Br of 2040 Gauss and an intrinsic coercive field strength 1% of 2380 oersted measured at room temperature. The apparent density (the quotient of weight and outer volume) was 4.87. As illustrated in Figure 1, the X-ray diagram showed that a magneto-plumbite phase was formed and, in addition, a small amount of x-Fez0z. The two phases had an X-ray density of 5.3, and therefore the relative volume of pores was Example 5 Starting with a mixture consisting 79.4 gms. (.4 mol) 01' barium carbonate (percent by weight of Ba=69.2) and 516.4 gms. (3.2 mols) of Fe2Oz (percent by weight of Fe=68.4). rods were extruded as described in Example 4, and the rods passed through an oven as described in The rods consisted of two phases, namely Example 1 at a speed of 5 mms. per minute, 01 a sintering time of 20 minutes. The sintering process took place in air at a temperature of 1330 C. The rods had a remanence Br of 1220 Gauss and an intrinsic coercive field strength I c of 1900 oersted, measured at room temperature.

Example 6 Rods were extruded from a mixture consisting 79.4 gms. (4 mol) of barium carbonate (percent by weight of Ba=69.2) and 580.9 grns. (3.7 mols) of Fe203 (percent by weight of Fe=68.4), and were sintered in a manner similar to that described in Example 5. The remanence of the rods was 1200 Gauss and the intrinsic coercive field strength 1% was 1950 oersted, measured al room temperature.

Example 7 A mixture consisting of 29.5 gms. (.2 mol) of pure strontium carbonate and 176.1 gms. 1.1 mols) of Fe203 (percent by weight of Fe=69.8) was dried at 200 C. and then ground for 4 hours with alcohol in a hall mill. After drying, the powder was pre-sintered at 1100 C. in air for about 3.5 hours. After a binder had been added as described in Example 1, rods were extruded from this mixture and, after being dried overnight, were sintered in an oven as described in the preceding examples. The sintering process took place in air at a temperature of 1280 C. at a speed of passage of 10 mms./min., corresponding to a sintering time of 10 min. The rods thus obtained had a remanence B1 of 2155 Gauss and an intrinsic coercive field strength 1% of 2500 oersted measured at room temperature. The apparent density (quotient of weight and outer volume) was 4.67. As shown in Fig. 2, the X-ray diagram showed that a magnetoplumbite phase was formed and that substantially no second phase was present. The X-ray density of the magnetoplumbite phase being about 5.2, the relative pore volume was about Example 8 Similarly, as described in Example 7, a mixture conssting of 14.8 grns. (.1 mol) of pure strontium carbonate and 112.1 gms. (.7 mol) of Fez0s (percent by weight of Fe=69.8) was formed into rods which were sintered in the above-described manner. The remanence B1 of the rods was 1535 Gauss, the intrinsic coercive field strength I c being 2800 oersted, measured at room temperature.

Example 9 A mixture consisting of 147.6 gms. (1 mol) of pure strontium carbonate (percent lpy weight of Sr=59.4) and 403.4 gms. (2.5 mols) of Fe2O3 (percent by weight of Fe=68.4) was extruded into rods and sintered as described in Example 4. The sintering process took place in air in an oven having a heating zone of from 3 to 5 cms. The temperature in the heating zone was 1200 C. and the speed of passage was 10 mms./min., corresponding to a sintering time of from 3 to 5 minutes. These rods had a remanence of 1555 Gauss and an irrtrinsic coercive field strength 1% of 3160 oersted, measured at room temperature.

Example 10 Example 11 In a similar manner as described in Example 10, a mixture consisting of 118.1 gms. (.8 mol) of pure strontium carbonate (59.4% by weight of Sr) and 510.9 gms. (3.2 mols) of F6203 (69.8% by weight of Fe) was extruded into rods, whch were subsequently sintered at 1280 C. in air in an oven having a heating zone of 10 cms. in length. The speed of passage was 10 mms./min. and the sintering time was 10 minutes. The rods thus sintered had a remanence Br of 1730 Gauss and an intrinsic coercve field strength I c of 3090 oersted measured at room temperature.

Example 12 Rods were extruded from a mixture consisting of 103.3 gms. (7 mol) of pure strontium carbonate (59.4% by weight of Sr) and 503.0 gms. (3.2 mols) of F6203 and were sintered in a similar manner as described in Example 11. The rods thus obtained had a remanence B1 of 1955 Gauss and an intrinsic coercve field strength I c of 2900 oersted measured at room temperature.

Example 13 Using a mixture consisting of 88.6 gms. (.6 mol) of pure strontium carbonate (59.4% by weight of Sr) and 479.0 gms. (3 mols) of F623 (69.8% by weight of Fe), rods were formed and sintered in a similar manner as described in Example 11. The rods thus obtained had a remanence B1 of 2100 Gauss and an intrinsic coercve field strength I c of 2800 oersted, measured at room temperature.

Example 14 A mixture consisting of 59.1 gms. (.4 mol) of pure strontiurn carbonate (59.4% by weight of Sr) and 580.0 gms. (3.6 mols) of F62O3 (68.4% by weight of Fe) was ground with alcohol in a ball mill for 15 hours. After drying, the mixture was extruded into rods as indicated in an Example 1. After drying overnight, the rods were sintered at 1270 C. in air in an oven having a heating zone of 10 cms. in length. The speed of passage was 5 mms. per minute, corresponding to a sintering time of 20 minutes. The rods thus obtained had a remanence B1 of 1290 Gauss and an intrinsic coercve field strength I c of 3080 oersted, measured at room temperature.

Example 15 A mixture consisting of 71.1 gms. (3 mol) of Pb02 (pro analysis) and 192.1 gms. (1.2 mols) of Fezs (69.8% by weight of Fe) was ground with alcohol in a ball mill for 4 hours. After drying, the powder was molded into pastilles, whch were pre-sintered first at 700 C. for 3 hours and then at 900 C. for 3 hours. The pastilles were then pulverized in a mortar, the powder being molded to form rods under a pressure of 1.5 tons/cmfi. The rods were sintered in an oven at 1150 C. in air. The length of the heating zone was from 3 to 4 cms. and the speed of passage was 10 mms./min. whch corresponded to a sintering time of from 3 to 5 minutes. The rods thus obtained had a remanence Br of 1630 Gauss and an intrinsic coercve field strength I c of 1000 oersted, measured at room temperature.

Example 16 A mixture consisting of 71.1 gms. (3 mol) of Pb0z (pro analysis) and 240.1 gms. (1.5 mols) of Fez0s (698% by weight of Fe) was ground with alcohol in a ball mill for 4 hours. After drying, the powder was molded into pastilles whch were presintered at first at 700 C. for 3 hours and then at 900 C. for 3 hours. Subsequently, they were pulverized in a mortar, the

powder being molded to form rods at a pressure of 1.5 tons/cmfi. The rods were sintered in air in an oven at a temperature of 1150 C. The length of the heating zone was from 3 to 5 cms. and the speed of passage was 10 mms./min., whch corresponded to a sintering time of 35 minutes. The rods thus obtained had a remanence B1 of 1420 Gauss and an intrinsic coercve field strength of 1685 oersted, measured at room temperature.

Example 17 A mixture consisting of 35.9 gms. (15 mol) of Pb0z (pro analysis) and 144.1 gms. (9 mol) of P6203 (69.8% by weight of Fe) was ground with alcohol in an iron ball mill for 4 hours. After being dred in an oven at 150 C., the pulverized mixture was presintered at 900 C. for 2 hours, whereafter a rod was molded therefrom under a pressure of 1.5 tons/cmfl. The rod was sintered at 1150 C. in air in an oven having a heating zone of from 3 to 5 cms. in length. The speed of passage was 10 mms./min., whch corresponded to a sintering time of from 3 to 5 minutes. The rod thus obtained had a remanence Br of1420 Gauss and an intrinsic coercve field strength I c of 1830 oersted, measured at room temperature. It should be noted that the sintering temperature in this and in the two preceding examples was maintained intentionally comparatively low in order to avoid an important loss of lead by evaporation.

Example 18 A mixture consisting of 18.74 gms. (.095 mol) of barum carbonate (pro-analysis), 0.5 grn. (005 mol) of calcium carbonate (pro analysis), and 95.55 gms. (.6 mol) of Fez0z (68.4% by weight of Fe) was ground with alcohol in a ball mill for 4 hours. After drying in air at C. presintering at 1000 C. in air took place for 2 hours. This was followed by renewed grinding with alcohol in a ball mill for 4 hours, the powder obtained being dried. A rod was molded from this powder at a pressure of 1.5 tons/cm. and sintered in an oven having a heatingzone of 10cms. in length. The sintering temperature was 1330 C. and the speed of passage 10 mms./min., whch corresponded to a sintering time of 10 minutes. The rod thus obtained had a remanence Br of 1680 Gauss and an intrinsic coercve field strength I c of 1110 oersted measured at room temperature. The

"-ray diagram, illustrated in Fgure 2, showed that a magnetoplumbite phase only was present. The body was composed mainly of crystals 2.5-5u in size. The largest crystals had dimensions of 8LL.

Example 19 In a marmer similar to that described in Example 18, a mixture conssting of 17.76 gms. (.09 mol) of barum carbonate (pro analysis), 1.001 gms. (.01 mol) of calcium carbonate (pro analysis) and 95.55 gms. (.6 mol) of Fez0s (68.4% by weight of Fe) was extruded into a rod and sintered. The sintered rod had a remanence Br of 1705 Gauss and an intrinsic coercve field strength I c of 1110 oersted measured at room temperature. The X-ray diagram showed that a magnetoplumbite phase only was present.

Example 20 In a manner similar to that described in Example 18, a mixture consisting of 15.79 gms. (.08 mol) of barum carbonate, 2.002 gms. (.02 mol) of calcium carbonate and 95.55 gms. (.6 mol) of Fe20z was molded into a rod, whch was sintered in air at 1290 C. in an oven having a heating zone of 10 cms. long and at a speed of passage of 20 mms./min., whch corresponded to a sintering time of 5 minutes. The rod had a remanence B1 of 1615 Gauss, an intrinsic coercve field strength I c of 1550 oersted and an apparent density d of 4.52 measured at room temperature. The X-ray diagram showed that the material obtained consisted substantially of a magnetoplumbite phase having an X-ray density of 5.2. The

Example 21 Sirnilarly, as described in the Examples 18, 19 and 20, a mixture consisting of 11.84 gms. (.06 mol) of barium carbonate, 4.004 gms. (.04 mol) of calcium carbonate and 99.55 gms. (.6 mol) of Fezs was ground with alcohol in a ball mill for 4 hours. After drying, this mixture was presintered in oxygen at 1000 C. for 2 hours, followed by grinding with alcohol in the ball mill for 4 hours. After drying, the mixture was molded at a pressure of 1.5 tons/cm. into a rod which was sintered at 1200 C. in an oven having a heating zone of cms. in length. The speed of passage was 10 mms./ min. which corresponded to a sintering time of 10 minutes. The rod thus obtained had a remanence B1 of 1660 Gauss and an intrinsic coercive field strengt-h I c of 1210 oersted, measured at room temperature.

Example 22 A mixture consisting of 59.5 gms. (.3 mol) of barium carbonate (69.2% by weight of Ba), 30.2 gms. (.3 mol) of calcium carbonate (40.0% by weight of Ca) and 500.0 gms. (3.1 mols) of Fe203 (69.4% by weight of Fe) was ground with alcohol in a ball mill for hours. After drying, a rod was molded from this mixture at a pressure of 1.5 tons/cm. and sintered in an oven at 1280 C. The oven had a heating zone 10 cms. in length, the rod being conducted through the oven at a speed of 10 mms./min. which corresponded to a sintering time of 10 minutes. The rod thus obtained had a remanence Br of 1595 Gauss and an. intrinsic coercive field strength 1% of 1890 oersted, measured at room temperature.

Example 23 In a manner similar to the descrbed in Examples 18 to 21, a mixture consisting of 11.81 gms. (.08 mol) of strontium carbonate (5.93% by weight of Sr), 2.002 gms. (.02 mol) of calcium carbonate and 95.55 gms. (.6 mol) of P6203 was ground, dried and extruded into a rod. The rod was sintered at 1280 C. in an oven having a heating zone of 10 cms. in length. The speed of passage was 10 mms./min., which corresponded to a sintering time of 10 minutes. The rod thus obtaned had a remanence Br of 2320 Gauss and an intrinsic coercive field strength of I c of 1395 oersted, measured at room temperature.

Example 24 Using a mixture consisting of 81.86 gms. (0.06 11101) of strontium carbonate (59.3% by weight of Sr), 4.004 gms. (0.04 mol) of calcium carbonate and 95.55 gms. (0.6 mol) of Fe203, a rod was manufactured by grinding, drying, melding and sintering in a similar manner as that described in Example 23. This rod had a remanence B: of 2120 Gauss and an ntrinsc coercive field strength I c of 1950 oersted, measured at room temperature.

Example 25 A mixture consisting of 59.05 gms, (.4 mol) of pure strontium carbonate dried at 100 C. (59.3% by weight of Sr) and 127.5 gms. (.8 mol) of Fez0s (69.8% by weight of Fe) was ground with alcohol in a ball mill for 7 hours. Aftel drying at 110 0, the mixture was presintered at 1000 C. in air for 4 hours. After renewed grinding as above and drying at 110 C. a rod was molded from the mixture at a pressure of 1.5 tons/ cm. and sintered at 1150 C in an oven having a heating zone of 4 cms. in length. The rod was passed through the oven at a speed of 10 mms./min, which corresponded to a sintering time of 4 minutes. The rod thus obtained had a remanence B1 of 1200 Gauss and an s intrinsic coercive field strength 1 0 of 3710 oersted, measured at room temperature.

Example 26 In a manner similar to that described in Example 25, a mixture consisting of 14.76 gms. (.1 mol) of strontium carbonate and 159.7 gms. (1 mol) of Fez03 was ground, dried and molded into a rod, and the rod was sintered at 1280 C. in an oven ha ving a heating zone of 10 cms. in length. The rod was passed through the oven at 2. speed of 10 mms./min, which corresponded to a sintering time of 10 minutes. The rod thus sintered had a remanence B1 of 1200 Gauss and an intrinsic coercive field strength I c of 3610 oersted, measured at room temperature.

Example 27 A mixture consisting of 44.3 gms. (.3 mol) of strontiurn carbonate (59.3% by weight of Sr), 30.2 gms. (.3 mol) of calcium carbonate (39.8% by weight of Ca) and 500.0 gms. (3.1 mols) of Fez0s (69.3% by weight of Fe) was ground with alcohol in a ball mill for 15 hours. After drying, a rod was pressed from the mixture at a pressre of 1.5 tons/cm. this rod being sintered at 1280 C. in an oven having a heating zone of 10 cms. in length. The rod was passed through the oven at a speed of 10 mms. per minute, which corresponded to a sintering time of 10 minutes. The rod thus sintered had a remanence B1 of 1645 Gauss and an intrinsic coercive field strength I c of 2460 oersted, measured at room temperature.

Example 28 A mixture consisting of 99.2 gms, (.5 mol) of barium carbonate (69.2% by weight of Ba), 73.8 gms. (.5 mol) of strontium carbonate (59.4% by weight of Sr) and 483.3 gms. (3 mols) of Fea0z (69.4% by weight of Fe) was ground with alcohol in a ball mill for 15 hours. After drying, a rod was molded from the mixture and sintered at a temperature of 1260 C. in an oven having a heating zone of 10 cms. in length. The rod was conducted through the oven at a speed of 5 mms./min, which corresponded to a sintering time of 20 minutes. The rod thus sintered had a remanence Br of 1665 Gauss and an intrinsic coercive field strength I c of 3235 oersted, measured at room temperature.

Example 29 In a manner similar to that described in Example 28, a mixture consisting of 99.2 gms. (.5 mol) of barium carbonate, 73.8 gms. (.5 mol) of strontium carbonate and 402.0 gms. (2.5 mols) of Fez0s was formed by grinding, drying and melding into a rod which was sintered at 1240 C. in an oven having a heating zone of 10 cms. in length. The rod was conducted through the oven at a speed of 10 mms./min, which corresponded to a sintering time of 10 minutes. The rod thus heated had a remanence Br of 1550 Gauss and an intrinsic coercive field strength 1 0 of 3410 oersted, as measured at room temperature.

Example 30 A mixture consisting of 41.7 gms. (.21 mol) of barium carbonate (69.2% by weight of Ba), 13.3 gms. (.09 mol) of strontium carbonate (59.4% by weight of Sr) and 435.5 gms. (2.7 mols) of FezOs (69.4% by weight of Fe) was forrned by grinding, drying and molding into a rod in a manner similar to that described in Example 28. This rod was sintered at 1300 C. in air in an oven having a heating zone of 10 cms. in length. The rod was passed through the oven at a speed of 10 mms./min, which corresponded to a sintering time of 10 minutes. The rod thus sintered had a remanence B1 of 1245 Gauss and an intrinsic coercive field strength I c of 2370 oersted, measured at room temeperature.

Example 31 A mixture consisting of 17.9 gms (.09 mol) of barium carbonate (69.2% by weght of Ba), 31.0 gms. (.2-1 mol) of strontiurn carbonate (59.4% by weight of Sr) and 435.5 gms. (2.7 mols) of Fe203 (69.4 by weight of Fe) was formed by grinding, drying and molding into a rod in a manner similar to that described in Example 28. Ths rod was sintered at 1300" C. in air in an oven having a heating zone of 10 cms. in lengtth. The rod was passed through the oven at a speed of 10 mms./min., whch corresponded to a sintering time of 10 minutes. The rod thus sintered had a remanence Br of 1340 Gauss and an intrinsic coercive field strength 1% of 2640 oersted, measured at room tcmpcrature.

Example 32 A mixture consisting of 70.2 gms. (.35 mol) of barium carbonate(69.2% by weight of Ba), 72.5 gms. (.3 mol) of lead dioxde Pbz (85.7% by weight of Pb) and 531.6 gms. (3.4 mols) of P6203 (69.3% by weight of Fe) was ground with alcohol in a hall mill for 15.5 hours. After drying, the mixture was presintered by slowly heating it up to at 900 C. and keeping it at this temperature for 2 hours. Subsequently, the mixture was molded into a rod which was sintered in air at 1145 C. in an oven having a heating zone of 4 cms. in length. The rod was conducted through the oven at a speed of mms.lmin. whch corresponded to a sintering time of 4 minutes. The rod thus sintered had a remanence Br of 1785 Gauss and an intrinsic coercive field strength I c of 2840 oersted, measured at room temperature.

Example 33 A mixture consisting of 79.4 gms. (.4 mol) of barium carbonate (per cent by weight of Ba=69.2) and 580.9 gms. (3.6 mols) of Fez0a (per cent by weight of Fe=68.4) was ground with alcohol in a hall mill for 15.5 hours and molded, after drying, into a pastille which was heatecl at 1300 C. in a carbon dioxde atmosphere for 1 hour. After coolng, the pastille was pulverised and ground with alcohol in a ball mill for 14 hours. The powder obtaned was again molded to form a pastille using a binder commonly used in ceramc industry such as nitrocellulose or polyvinyl-alcohol. The pastille was pulverised, a rod being molded from the resultant grains at a pressure of 12 tons/crnfl. Ths rod had a remanence B1'- of 1260 Gauss and an intrinsic coercive field strength I c of 700 oersted, measured at room temperature.

The X-ray diagram (Fig. 3) showed that the material consisted almost solely of hexagonal crystals having an a-axis of about 5.8 A. and a c-axis of about 33 A. The crystals have approximately the composition BaFers0zz. A small portion of the iron was present therein in the bivalent state, which was connected with the manufac ture of the material by heating in an oxygen deficient atmosphere containing a small percentage of oxygen, viz. a carbon dioxde atmosphere.

Example 34 A mixture consisting of 39.5 gms. (.2 mol) of barium carbonate and 191.7 gms. (1.2 mols) of Fez0a was grouncl with ethanol in an iron hall mill for 4 hours, dried in a drying oven after evaporation of the ethanol and subsequently presintered in air at 1000 for 4 hours. After cooling, the presintered product was again ground with ethanol in the hall mill for 4 hours and, after evaporation of the ethanol, was molded into rods at a pressure of 1.5 tons/cmfi. Rods were manufactured in exactly the same manner from a mixture consisting of 29.5 gms. of strontium carbonate and 191.7 gms. of Fe203.

Three rods of the first mixture and three rods of the second mixture were sintered in diiferent manners as indicated in the table below. The sintering process took place in oxygen, but may alternatively take place in air. The values for the remanence Br and the intrinsic co- BELO.G F930; S1O.6Fe20s Sintering tem- Rod No. perature and sinterng time B (in 1 a (in B.(in 1 e (in Gauss) Oersted) Gauss Oersted) Example 35 The table below serves to illustrate the influence of the composition of the initial material upon the result of thesintering process. The table shows that very high I c values (measured at room temperatures) are obtainable with different compositions, provided that the sintering temperatnre and the time of sintering conform with the composition. In the tests of the table below, the duration of the sintering process was, in each exarr1ple 4 hours.

Rods were manufactured in accordance with three difierent procedures, using a mixture conssting of 9.9 gms. (.05) of barium carbonate and 46.9 gms. (0.20 mol) ofnatural magnetite, Fe3O4.

According to the first procedure, the mixture whch had been ground in a hall mill for 18 hours was presinte red at 900 C. for three hours and, after cooling, again ground in the hall mill for 18 hours and then molded to form a rod at a pressure of 1.5 tons/cmfl. The rod was subsequently sintered at 1200 C. for 2 hours. Accordingto the second procedure, the mixture of barium carbonate and magnetite, after being ground for 18 hours in a hall mill, was molded directly under a pres sure 1.5 tons/cm. to form a rod whichwas sintered at 1200 C. for 2 hours.

According to the third procedure, the mixture of barium carbonate and magnetite was ground na ball mill for half an hour only and subsequently molded directly under a pressure of 1.5 tons/cm. to form a rod which was sintered at 1200 C. for 4 hours. The measured values for the remanence B1 and the intrinsic coercive field strength I c, measured at room temperatures, are given in the table below.

Roti No. B. (in m (in Gauss) Oersted) While the invention has been declared with specific examples and applcations thereof, we do not desire to be limited thereto as other modifications of the invention will be readily pparent to those skilled in ths art with-- out departing from the spirit and scope of the invention as defined in the appendical claims.

What we claim is:

1. A method of making a permanent magnet having an intrinsic coercive force (1 0) of at least 700 oersted and a remanence (Br) of at least 1200 gauss comprising, the steps of compacting intoa body of a desired shape, finelydivided material having a composition selected from the group consisting of MFerzrg and MF618021 in which M is a metal se1ected from the group consisting of Ba, Sr and Pb, said material being obtained -by heating a mixture of an oxide selected from the group consisting of barium, strontium and lead oxides and Fez0a in a mol ratio of between about 1:2 and 1:10 at a temperature between about 900 C. and 1100 C.; heating said compacted body to a temperature between about 900 C. and 1450 C. to sinter the same into a highlycoherent body; and magnetizing said latter body.

2. A method of making a permanent magnet having an intrinsic coercive force (I c) of at least 700 oersted and a remanence (Br) of at least 1200 gauss comprising, the steps of mixing in a finely-divided form at least one oxide selected from the group consisting of barium, strontium andlead oxides and P6203 in a mol ratio of between about 1:2 and 1:10; compacting said fineiy-divided mixture into a body of desired shape; heating said compacted body at a temperature between about 900 C. and 1450 C. to sinter the same into a highly-coherent body having a composition selected from the group consisting of MF61219 and MFem0zv, M being at least one of the metals se1ected from the group consisting of barium, strontium and lead; and magnetizing said iatter body.

3. A method of making a permanent magnet having an intrinsic coercive foree (1%) of at least 700 oersted and a remanence (Br) of at least 1200 gauss comprising, the steps of compacting into a body of a desired shape, finely-divided material having a composition se1ected from the group consisting of MFe12O19 and MFe1s021 in which M is a metal seieeted from the group consisting of Ba, Sr and Pb; heating said compacted body at a temperature between about 900 C. and 1450 C. to sinter the same into a highly-coherent body, and magnetizing said latter body.

4. A method of making a permanent magnet having an intrinsic coercive force (I c) of at least 700 oersted and a remanence (Br) of at least 1200 gauss comprising, the steps of compacting into a body of desired shape, finely-divided material having a composition selected from the group consisting of MFem0rs and MFe1s021 in which M is a metal selected from the group consistng of Ba, Sr and Pb; heating said compacted body at a temperature between about 1100 C. and 1450 C. to sinter the same into a highiy-coherent body; and magnetizing said latter body.

5. A method of making a permanent magnet having an intrinsic coercive force (1%) of at least 700 oersted and a remanence (Br) of at least 1200 gauss comprising, the steps of mixing in a finely-divided form at least one oxide selected from the group consisting of barium, strontium and lead oxides and Fez0a in a mo1 ratio of between about 1:2 and 1:10; compacting said mixture into a body of desired shape; heating said compacted body at a temperature between about 1100 C. and 1450 C. to sinter the same into a highiy-coherent body having a composition seleeted from the group consisting of MF612019 and MFGISC27, M being at least one of the metals selected from the group consisting of barium, strontium and lead; and magnetizing said 1atter body.

6. A method of making a permanent magnet having an intrinsic coercive force (I c) of at least 700 oersted and a remanence (1) of at least 1200 gauss comprising, the steps of mixing in a finely-divided form at least one oxide from the group consisting of barium, strontium and lead oxides and Fe20a in a mol ratio of between about 1:2 and 1:10; heating said mixture at a temperature between about 900 C. and 1100 C., finely-dividing said heated mixture; compacting said later mixture into a body of desired shape; heating said mass at a temperature between about 1100 C. to 1450 C. to thereby sinter said mass and form a highly-eoherent body of material having a composition selected from the group consisting of MF61219, MFe1s0zv, M being at least one of the metals selected from the group consisting of Ba, Sr and Pb; and magnetizing said 1atter body.

7. A method of making a permanent magnet having an intrinsic coercive force (1%) of at least 700 oersted and a remanence (Br) of at least 1200 gauss comprising, the steps of compacting into a body of desired shape finely-divided material having a composition of BaFeua0zv, said material being obtained by heating a mixture of barium oxide and Fez0z, in a mol ratio of between about 1:2 and 1:10 at a temperature between about 900 C. and 1100 C. in an oxygen-deficient atmosphere; heating said compacted mass at a temperature between about 1100 C. and 1450 C. to sinter the same into a high1y coherent body; and magnetizing said 1atter body.

8. A method of making a permanent magnet having an intrinsic coercive force (I c) of at least 700 oersted and a remanence (Br) of at least 1200 gauss comprising, the steps of compacting into a body of desired shape finely-dvided materia1 having a composition B21F612019, said material being obtained by heating a mixture of a barium oxide and P6203 in a mol ratio of between about 1:2 and 1:10 at a temperature between about 900 C. and 1I00 C. heating said compacted body at a temperature between about 1100 C. and 1450 C. to sinter the same into a highly-coherent body; and magnetizing said latter body.

9. The method of claim 8 in which the barium oxide and P6203 are in a mol ratio of about 125.5.

10. A method of making a permanent magnet having an intrinsic coercive force (1 0) of at least 700 oersted and a remanence (Br) of at least 1200 gauss comprising, the steps of compacting into a body of desired shape, finely-divided material having a composition SrFem01s, said material being obtained by heating a mixture of a strontium oxide and Fez0s in a mol ratio between about 1:2 and 1:10 at a temperature between about 900 C. and 1100 C.; heating said compacted body at a temperature between about 1100 C. and 1450 C. te sinter the same into a highly-coherent body; and magnetizing said latter body.

11. A method of making a permanent magnet having an intrinsic coercive force (1%) of at least 700 oersted and a remanence (Er) of at least 1200 gauss comprising, the steps of compacting into a body of desired shape, finely-divided material having a composition PbF612019, said material being obtained by heating a mixture of a lead oxide and Fez0a in a mo1 ratio between about 1:2 and 1:10 at a temperature between about 900 C. and 1100 C.; heating said compacted body to a temperature between about 1100 C. and 1450 C. to sinter the same into a highly-coherent body; and magnetizing said 1atter body.

12. As a permanent magnet, a magnetized highly-coherent sintered body consisting essentially of a compact mass of hexagonal crystals of a material seiected from the group consisting of MFe1219 and MFem0zv in which M is at least one metal se1ected from the group consisting of barium, strontium and lead, said magnet having an intrinsie coercive force (1 0) of at least 700 oersted and a remanence (Br) of at least 1200 gauss.

13. As a permanent magnet, a magnetized highly-coherent sintered body consisting essentially of a compact mass of hexagonal crysta1s of a material consisting of MFerz0m in which M is at least one metal selected from the group consisting of barium, strontium and lead, said magnet having an intrinsic coercive force (1 0) of at least 700 oersted and a remanence (Br) of at least 1200 gauss.

14. The permanent magnet of claim 13 in which M is a mixture of barium and strontium.

15. As a permanent magnet, a magnetized highly-coherent sintered body consisting essentia1ly of a compact mass of hexagonal crystals of a material selected from the group consisting of MF612O19 and MFers0zr in which M is at least one meta1 selected from the group consisting 13 of barum, strontum, and lead and calcium in an atomc fracton up to 0.4 of sad metal, sad magnet having an intn'nsc coercive force (1 0) of at least 700 oersted and a remanence (Br) of at least 1200 gauss.

16. As a permanent magnet, a highly-coherent sintered body consistng essentally of a compact mass of hex agonal crystals of BaFeu0m, sad magnet havng an intrnsc coercive force (I c) of at least 700 oersted and a remanence (Br) of at least 1200 gauss.

17. As a permanent magnet, a magnetzed hghy-coherent sntered body conssting essentally of a compact mass of hexagonal crystals of BaFers0z7, sad magnet having an ntrnsc coercve force (1 0) of at least 700 oersted and a remanence (Br) of at =least 1200 gauss.

18. As a permanent magnet, a magnetzed hghly-coherent sntered body consisting essentially of a compact mass of hexagonal crystals of SrFerzm, sad magnet hav- 14 ing an intrinsic coercive force (1 0) of at least 700 oersted and a remanence (Br) of at least 1200 gauss.

19. As a permanent magnet, a magnetized highly-cohercnt sntered body conssting essentially of a compact mass of hexagonal crystals of PbF12O19 sad magnet having an intrnsic coercive force (I c) of at least 700 oersted and a remanence (Br) of at least 1200 gauss.

References Cited in the file of ths patent Mellor: Comprehensive Treatse on Inorganic and Theoretcal Chemstry, vol. 13, page 914, Longmans Green & Co. N. Y. (1934). Copy in Science Lbrary.

Journal of Amercan Chem. Soc., v. 68, October 1946, pages 2085-2096. Copy in Sc. Lib.

Proceedngs of the Instituton of Electrcal Engineers, vol. 97, part II, No. 56, April 1950, pages 248-249 pertinent. Copy Div. 64. 

1. A METHOD OF MAKING A PERMANENT MAGNET HAVING AN INTRINSIC COERCIVE FORCE (IHC) OF AT LEAST 700 OERSTED AND A REMANENCE (BR) OF AT LEAST 1200 GAUSS COMPRISING, THE STEPS OF COMPACTING INTO A BODY OF A DESIRED SHAPE, FINELY-DIVIDED MATERIAL HAVING A COMPOSITION SELECTED FROM THE GROUP CONSISTING OF MFE12O19 AND MFE18O27 IN WHICH M IS A METAL SELECTED FROM THE GROUP CONSISTING OF BA, SR AND PB, SAID MATERIAL BEING OBTAINED BY HEATING A MIXTURE OF AN OXIDE SELECTED FROM THE GROUP CONSISTING OF BARIUM, STRONTIUM AND LEAD OXIDES AND FE2O3 IN A MOL RATIO OF BETWEEN ABOUT 1:2 AND 1:10 AT A TEMPERATURE BETWEEN ABOUT 900* C. AND 1100* C.; HEATING SAID COMPACTED BODY TO A TEMPERATURE BETWEEN ABOUT 900* C. AND 1450* C. TO SINTER THE SAME INTO A HIGHLYCOHERENT BODY; AND MAGNETIZING SAID LATTER BODY. 